Markers, including broad-nib markers, are especially attractive to children for the intense markings producible with letter pressure. Unfortunately, the intensity of the markings is rapidly depleted without sealed protection of the marking nib during periods of non-use. Marking nib protection is accomplished in the prior art by caps which fit tightly upon the marker casing. Caps are problematic for children who tend to put in them in their mouths and thereby risk choking or ingestion. And even older children have difficulty removing and replacing tight fitting caps. Caps can also be easily lost, thereby rendering the accompanying marker useless in a very short period of time.
Ink impregnated marking and writing instruments, commonly known as "markers" have a fabric or polymeric ink carrier, one end of which forms the marking or writing tip or nib. The term "markers" as used herein includes all such instruments having a fabric or fibrous or polymeric ink carrier and nib, including for example but not limited to felt-tipped pens, highlighters, permanent and non-permanent markers, grease pens, and children's' markers. The ink-impregnated carrier may serve as the ink cartridge, or be in contact with a fluid ink reservoir, with the ink flowing to the marking nib by capillary action.
Exposure of the marking nib to the atmosphere causes rapid evaporation of ink from the carrier at the nib, and resultant drying of the nib material which blocks the flow of fresh ink to the nib. This is why it is necessary to tightly cover the marker nib of marking instruments.
Most markers have plastic caps which securely fit over the nib in contact with the instrument shaft or body. The cap is thus necessarily detachable from the instrument and therefore easily lost. Loose marker caps are an obvious danger to small children who may swallow them. With most markers it is necessary to fly snap the cap onto the shaft to insure adequate sealing of the nib from the atmosphere. This is also a problem for children not strong enough to either remove the cap or replace it tightly.
U.S. Pat. No. 4,269,525 describes a retractable marker pen with a shield mechanism 16 in the form of a flexible disk positioned immediately at the writing tip opening of the barrel of a pen, to flexibly open and close as the pen tip emerges and retracts relative to the opening. In practice, however, the design and configuration of the flexible seal is critical to its operation and, most importantly, effectiveness as an atmospheric barrier. For example, each slit in the membrane which forms the seal provides an additional path for air to enter the marker or pen barrel which houses the marker nib, and creates a plurality of pie-shaped flaps which with wear can become too flexible and fail to return to the sealed position with adjacent flaps. Each flap must seal on both its edges to adjacent flaps. Longevity and durability of the seal is also a major concern. The seal must remain intact throughout long periods of non-use, and survive thousands of extension/retraction cycles. The seal design of the '525 patent is deficient in these respects, having multiple radial slits which increase the number of air paths to the interior of the pen, and being positioned at the very tip of the writing instrument where it is fully exposed to mechanical and atmospheric degradation.
U.S. Pat. No. 5,092,701 describes an adult writing pen with a complex three-disk seal assembly in the front pen socket. The seal assembly is opened by a sliding sleeve which surrounds the writing nib. This design also suffers from its own complexity, in particular the difficulty of holding the three disks of the seal assembly in the described misalignment. Also, the described Y-shaped slits in each of the disks are not ideally suited to form and maintain a tight seal. In fact, every incision in a member which defines relatively movable parts of the seal weakens the sealing force at the mating surfaces of the parts. In other words, each of the parts becomes more flexible, exerting less force against adjacent parts. Seals with multiple slits are also more susceptible to mechanical degradation and damage, as may be induced by the repeated linear and rotational movements of the writing nib and the sleeve against the seal.
There is thus absent from the prior art a retractable capless marker which has a highly effective marker nib seal of a relatively simple design, which eliminates the need for a separate cap, and which is ideally suited for use by children.